1. Technical Field
Embodiments of the invention relate generally to flexible circuit devices and more particularly, but not exclusively, to structures for providing a ground potential with a flexible printed circuit.
2. Background Art
In various laptop, Ultrabook, smartphone and other designs, it is common to provide electrical signaling between a printed circuit board (PCB), or other rigid circuit device, and flexible circuitry such as that of a flexible printed circuit (FPC) or a flexible flat cable (FFC). Direct soldering of FPC or FFC signal traces to a PCB is a common usage in these platforms.
FIG. 1A shows a cross-sectional view of a conventional system 100 wherein a FPC 106 includes a signal line 110 coupled to exchange a signal between PCBs 112, 114. Signal line 110 is directly coupled at ends 120, 122 of FPC 106 to respective interface hardware (not shown) of PCBs 112, 114. A floating ground plane 108 of FPC 106 provides shielding for the signal exchanged via signal line 110. In system 100, signal line 110 is closer than floating ground plane 108 to PCBs 112, 114, where a maximum height of floating ground plane 108 above PCBs 112, 114 is greater than a corresponding height of signal line 110. Floating ground plane 108 is not tied to a reference potential on PCB 112 or PCB 114, and is allowed to “float” electrically, which, while accommodating flexibility, is susceptible to radio frequency interference (RFI) issues.
FIG. 1B shows a cross-sectional view of another conventional system 102 wherein a FPC 136 includes a signal line 140 and a ground line 142 coupled to exchange, respectively, a signal and a reference potential between PCBs 132, 134. Signal line 140 and ground line 142 are each directly coupled at ends 150, 152 of FPC 136 to respective interface hardware (not shown) of PCBs 132, 134, where a maximum height of signal line 140 above PCBs 112, 114 is greater than or equal to a corresponding maximum height of ground line 142. A floating ground plane 138 of FPC 136 provides shielding for the signal exchanged via signal line 140, where a maximum height of floating ground plane 138 above PCBs 132, 134 is greater than a corresponding maximum height of signal line 140. Floating ground plane 138 is not directly coupled to any grounding contact of either PCB 132 or PCB 134.
FIG. 1C shows a cross-sectional view of another conventional system 104 wherein a FPC 170 includes a signal line 174 coupled to exchange a signal with a PCB 160. Signal line 174 is directly coupled to a connector 162 of PCB 160. A floating ground plane 172 of FPC 170 cannot provide complete shielding for the signal exchanged via signal line 174, where signal line 174 is closer than floating ground plane 172 to PCB 160. A connector shield 164 of PCB 160 extends over and around an end of FPC 170 at the point of coupling to connector 162. Floating ground plane 172 is not directly coupled to any grounding contact of PCB 160 (or any other rigid circuit device).
Flexible shielding structures disposed over signal lines (such as those variously illustrated by floating ground planes 108, 138, 172) are reaching the limits of their effectiveness as signaling technologies continue to trend toward higher frequencies and lower signal power. For example, electromagnetic interference (EMI)—such as radio frequency interference (RFI) resulting from signal reflection 180 near connector shield 164 and other such structures—will have an increasingly significant effect on signal communication. As successive generations of electronic devices continues to provide greater integration, higher signal frequency, lower signal power, etc., there is expected to be an increasing value placed on incremental improvements in how signal shielding is provided with flexible circuitry.